Black Holes Burping

[Forum Explorer]

So apparently black holes burp (http://www.bbc.com/news/science-environment-35237863)

This seems kinda like big news, though I'm not an astrophysist so what exactly this means is beyond me. But aren't Black Holes supposed to be inescapable? What does gas escaping one mean? How is it escaping the black hole in the first place? Does anyone even know the answers to those questions?

[Seppl]

The matter that gets expelled never fell into the black hole, it was just somewhere in the vicinity. When a lot of matter at once is falling into a black hole, things can get pretty crowded. The matter will form a spinning disc around the black hole before falling into it. But there is also a lot of gravitational energy that gets released in the process that has to go somewhere. Same for angular momentum. Therefore, these accretion discs are violent, hot places. The exact mechanisms is still unknown¹ but somehow enough of this energy can get transfered to a part of the falling matter, violently throwing it out again, before it is consumed by the black hole.



¹: That is one of the reasons, why the new observations are significant. They can be compared to the predictions made by the different models that have been proposed.

[Sith_Happens]

The exact mechanisms is still unknown

Is it not just a statistical mechanics thing, kind of like volatility in liquids?

[Seppl]

Is it not just a statistical mechanics thing, kind of like volatility in liquids?
You can certainly describe it like that and come to the conclusion, that these expulsion events must happen. But it does not tell you how they happen. As far as I know, there are several competing theories as to how exactly momentum gets transferred from the disk (or even the black hole itself? We do not know for certain!) to the ejected matter.

[Sith_Happens]

Well vapor pressure just happens due to the liquid particles not having uniform energy; some of them get going fast enough to break the surface and enter the gas phase. Seems like the same principle should apply here; the accretion disk is bumping and grinding into itself and swapping energy back and forth between its constituent particles, occasionally some of them should end up with enough at once to get out.

[Trixie]

But aren't Black Holes supposed to be inescapable?

No. In fact, there is absolutely nothing stopping you from escaping black hole. Well, at least under classical physics, relativity has this pesky limit of needing to move faster than C once you get too close - region near black hole where you cross the C barrier is called Event Horizon and is what layman would call "inescapable" part. But, here is the thing, before you cross it, escape is perfectly possible, and the black hole apparently ejects some orbiting matter in new kind of interaction before it crosses it. In fact, this is how black holes "evaporate", lose mass by events happening right on the edge of EH, where tiny bit of energy falls into black hole and another bit that would normally fade away escapes it, carrying away tiny part of its mass (e = mc2)

To make it even more complicated, there are (theoretically possible) charged black holes without Event Horizon. Meaning, you can escape them easily. But, no one observed those and since they happen to break all laws of physics (and to make it even better, they break different laws in different ways) astrophysicists hate them and the (shaky) consensus is they don't exist. But then again, Dirac and his theoretical positively charged electrons were laughed at at first too, sooo...

[Forum Explorer]

No. In fact, there is absolutely nothing stopping you from escaping black hole. Well, at least under classical physics, relativity has this pesky limit of needing to move faster than C once you get too close - region near black hole where you cross the C barrier is called Event Horizon and is what layman would call "inescapable" part. But, here is the thing, before you cross it, escape is perfectly possible, and the black hole apparently ejects some orbiting matter in new kind of interaction before it crosses it. In fact, this is how black holes "evaporate", lose mass by events happening right on the edge of EH, where tiny bit of energy falls into black hole and another bit that would normally fade away escapes it, carrying away tiny part of its mass (e = mc2)

To make it even more complicated, there are (theoretically possible) charged black holes without Event Horizon. Meaning, you can escape them easily. But, no one observed those and since they happen to break all laws of physics (and to make it even better, they break different laws in different ways) astrophysicists hate them and the (shaky) consensus is they don't exist. But then again, Dirac and his theoretical positively charged electrons were laughed at at first too, sooo...

Sorry, I assumed that when the article was talking about matter escaping the event horizon, which is why I was so confused.

Gotta love violations of our current understandings of reality. Those are the best. :smallbiggrin:


The matter that gets expelled never fell into the black hole, it was just somewhere in the vicinity. When a lot of matter at once is falling into a black hole, things can get pretty crowded. The matter will form a spinning disc around the black hole before falling into it. But there is also a lot of gravitational energy that gets released in the process that has to go somewhere. Same for angular momentum. Therefore, these accretion discs are violent, hot places. The exact mechanisms is still unknown¹ but somehow enough of this energy can get transfered to a part of the falling matter, violently throwing it out again, before it is consumed by the black hole.



¹: That is one of the reasons, why the new observations are significant. They can be compared to the predictions made by the different models that have been proposed.

I see, that makes a lot more sense, thank you.

[Seppl]

Well vapor pressure just happens due to the liquid particles not having uniform energy; some of them get going fast enough to break the surface and enter the gas phase. Seems like the same principle should apply here; the accretion disk is bumping and grinding into itself and swapping energy back and forth between its constituent particles, occasionally some of them should end up with enough at once to get out.Yes, but how does this "grinding" take place? We are still talking about a dilute gas that is just running around in circles. There is not enough interaction going on to explain the observations, unless you postulate some mechanism beyond conventional viscous stress. There are several competing ideas of how this might happen. Among the most popular ones are the idea of additional turbulence within the disk (but where do they come from?) and (as always in science :smalltongue: ) magnetic forces.

[Bulldog Psion]

It's caused by starships trying to escape from the gravity well.

[MLai]

In fact, this is how black holes "evaporate", lose mass by events happening right on the edge of EH, where tiny bit of energy falls into black hole and another bit that would normally fade away escapes it, carrying away tiny part of its mass (e = mc2)
I thought evaporation via Hawking radiation works by the fact that the energy/mass falling into the BH is negative in sign? Because virtual pairs do not come from the BH, so 1 particle of a pair escaping shouldn't really affect the BH directly, except for the fact that the twin particle it eats is negative.

[factotum]

Because virtual pairs do not come from the BH, so 1 particle of a pair escaping shouldn't really affect the BH directly, except for the fact that the twin particle it eats is negative.

Thing is, whichever one of the virtual pair falls in to the black hole, you end up with a particle existing where none existed before, and the energy for that has to come from *somewhere*--in this case, it comes from the black hole.

[Sith_Happens]

(Hey, I remember this thread!:smalltongue:)


Yes, but how does this "grinding" take place? We are still talking about a dilute gas that is just running around in circles. There is not enough interaction going on to explain the observations, unless you postulate some mechanism beyond conventional viscous stress. There are several competing ideas of how this might happen. Among the most popular ones are the idea of additional turbulence within the disk (but where do they come from?) and (as always in science :smalltongue: ) magnetic forces.

The way I usually see accretion disks described they sound more than dense enough for kinetic interaction to take place, not to mention radiation exchange.

[Yora]

I thought evaporation via Hawking radiation works by the fact that the energy/mass falling into the BH is negative in sign? Because virtual pairs do not come from the BH, so 1 particle of a pair escaping shouldn't really affect the BH directly, except for the fact that the twin particle it eats is negative.

No, it doesn't matter which particle falls in and which one escapes. The effect is always the same.

The energy that is used to create the particle pair has to come from somewhere. One oddity of the universe is that the energy of a perfect vacuum is not 0, as one would expect, but slightly positive. If empty space had 0 energy, the universe would be a completely different place.

In quantum mechanics, subatomic particles have no fixed position until they interact with something. Before that there are just probabilities for the odds to find the particle in any given space. If one point in space spontaneously creates a pair of virtual particles, these particles might be found in two positions a small distance away from that point. So it is possible that a point just very slightly inside the event horizon of a black hole creates a pair of particles and one of them is far enough away from that point to be now outside the event horizon.
Since its companion is lost to the black hole, it can't instantly annihilate and might fly off into space and escape. Though even if it would fly off into a random direction the chance for that would only be 50%, with the other 50% being heading directly into the black hole. But I think because of the spacetime warping, the chance to end up in the black hole is actually much higher than 50%.

So you need to have a spontaneous creration of a particle pair extremely close to the event horizon, one of them must end up in a place that is outside the event horizon, and it then has to fly off into a direction that doesn't lead into the black hole. Obviously the chance for all that to happen is very low and the loss of energy/mass from such an event is very small, so it takes an unbeliveably long time for a star-mass black hole to evaporate this way.

However, if the particle pair is created from a point in space just slightly outside the event horizon and one of the two particles fall into the black hole, wouldn't the black hole also gain mass this way? And given that the event horizon is slightly curved, wouldn't such event take place on the outer side of the horizon happen more often than on the inside? In that case the process would result in a gain of energy and mass for the black hole. And if the chance for any particle to escape is lower than 50%, it would be even stronger.
Assuming Hawking and all the black hole physicists didn't make such an obvious oversight, why does the process cause the black hole to get smaller instead of bigger?

[Sith_Happens]

However, if the particle pair is created from a point in space just slightly outside the event horizon and one of the two particles fall into the black hole, wouldn't the black hole also gain mass this way? And given that the event horizon is slightly curved, wouldn't such event take place on the outer side of the horizon happen more often than on the inside? In that case the process would result in a gain of energy and mass for the black hole. And if the chance for any particle to escape is lower than 50%, it would be even stronger.
Assuming Hawking and all the black hole physicists didn't make such an obvious oversight, why does the process cause the black hole to get smaller instead of bigger?

I would assume that the amount of energy (and therefore mass) lost by the black hole to the overall process is simply greater than that of the particle that fell in. Why? There might be a direct causative reason but at base it's because otherwise you'd be permanently violating conservation of energy which is of course a huge no-no.

[halfeye]

You can certainly describe it like that and come to the conclusion, that these expulsion events must happen. But it does not tell you how they happen. As far as I know, there are several competing theories as to how exactly momentum gets transferred from the disk (or even the black hole itself? We do not know for certain!) to the ejected matter.

There's a thing that happens with frisbee (TM) and other similar disks. If you throw one with a lot of spin, but a fairly slow forward speed, then at a certain point, predictable to the thrower with a lot of practice, they almost halt, then wobble somehow, and a lot of the rotational energy is transferred into velocity. This can make catching one a lot trickier that you might naively expect.

[Yora]

I tried to do some quick online searching (which of course usually gets you few good results), and it appears that the "quantum tunneling interpretation" seems to be considered valid.
And it seems that gravity does not cause the chance for a particle to escape to be lower than 50%. In fact, because the event horizon is curved, the chance to escape is always greater than 50%. (Draw any curved line through the center of a circle: You will always a larger area outside the curve than inside the curve.) The smaller the diameter of the event horizon, the greater the curvature (football looks round, Earth looks almost flat), and that explains why the process accelerates as the black hole is getting smaller.

But somehow I feel like this sounds waaay too simplistic and doesn't include mention of many other things that appear to be super important.

One source I found said that it doesn't matter which particle falls into a black hole and which one escapes, as the one that falls in "appears to have negative energy to an outside observer". But I don't have the slightest clue what "apparent negative energy" could mean. (And it wasn't elaborated in the video.) This seems rather important and doesn't exist in my "simplified popular science interpretation", so I rather doubt I actually understand the process.

This (http://physics.stackexchange.com/questions/30597/black-holes-and-positive-negative-energy-particles) seems to go into it someway, but it's way too late for me to make any complex thinking today.

[weaseldust]

I tried to do some quick online searching (which of course usually gets you few good results), and it appears that the "quantum tunneling interpretation" seems to be considered valid.

You might also appreciate this: Hawking radiation is not produced at the black hole horizon (http://backreaction.blogspot.co.uk/2015/12/hawking-radiation-is-not-produced-at.html)


But I don't have the slightest clue what "apparent negative energy" could mean.

As far as I understand, virtual particles are just a kind of accounting fiction you need to make a particular kind of calculation work. The energy of any particular virtual particle will be whatever is needed to make energy conserved overall (and obey other rules), even if the value wouldn't make sense for a real particle. In this case, it comes out negative, but that's the same thing as saying the black hole loses energy when it emits a particle (at least, from the perspective of someone who observes it to be emitting particles, which should be anyone outside the black hole - I'm not sure what someone inside observes).

[Sith_Happens]

You might also appreciate this: Hawking radiation is not produced at the black hole horizon (http://backreaction.blogspot.co.uk/2015/12/hawking-radiation-is-not-produced-at.html)

So... Giant relativistic vacuum orbitals, more or less, and Hawking radiation comes from something at least analogous to particles tunneling out of the black hole. Makes about as much sense as anything else QM-related.

[Yora]

So I was right that it doesn't matter where the particles originate, just that one falls in and the other escapes? I underestimated the distance at which it happened by several orders of magnitude, but I was on to an actual error in the common description. That makes me feel quite smart now. :smallbiggrin:

Though I still have no idea what that negative energy is about and how it makes a black hole evaporate. But still... :smalltongue:

[RCgothic]

Hawking radiation aside, my understanding is that black holes 'burping' likely comes from the accretion disc. As mentioned, these are hot, energetic regions orbiting very fast. Hot particles are a plasma, and fast-moving plasma generates monumental magnetic fields. But the magnetic forces aren't uniform, so the field lines get tangled up, forming dense pockets of extremely high magnetic energy. These eventually snap back into shape, and as they do so the changing magnetic field releases enough energy into the plasma to eject some of it.

[Lord Torath]

It's really less "burping", and more "twirling the spaghetti on your fork quickly enough that you send some of it flying" or "dropping a meatball off your spoon and onto your shirt just before it reaches your mouth". The "belches" are never actually consumed by the black hole, but are expelled before they can cross the event horizon. It's clumsy eating, not poor manners.

[Sith_Happens]

So I was right that it doesn't matter where the particles originate, just that one falls in and the other escapes? I underestimated the distance at which it happened by several orders of magnitude, but I was on to an actual error in the common description. That makes me feel quite smart now. :smallbiggrin:

Though I still have no idea what that negative energy is about and how it makes a black hole evaporate. But still... :smalltongue:

The key thing to remember is that the reason they're called "virtual" particles is because the one that fell into the black hole never really existed in the first place. The net physical process is more like some of the black hole's energy tunneling out and becoming a particle.